Rigging issues- specifically errors when weight painting
I suspect you are trying to assign from Object Mode. Try again from Pose Mode?

First make sure the bones you are wanting to work on are visible and selected in Pose Mode. To make things clearer I usually hide what I am not working on. Select the Mesh Object and then the Armature - still in Pose Mode - and press CTRLP and select Armature deform. Select the Mesh Object and go into Weight Paint....



Under the Weight Menu select Assign Automatic from Bones